First of all, any idea to be compatible with Islam, it has to conform all the principles and points mentioned about that idea in Islam. Then the idea may be expanded without making any collision with those basic points. Only then the idea can be considered compatible with Islam. But still none can claim that this IS the idea/definition of Islam, it is just compatible. And the same principle will apply to this question- whether "Pantheism", which is a theory about the nature of God, is compatible with Islam or not.
So let's start with the idea/definition of God in Islam. You have already asked a question in this site regarding this- Who is God? Do all Muslims agree on one definition?. And there are some very good, well organized discussion about this matter. To summarize, there are many verses of Quran and Ahadith that mention the attributes of Allah- the most important being Surat Al-'Ikhlas http://quran.com/112 (you can check different translations here). Another important source is the 99 names of Allah- http://en.wikipedia.org/wiki/99_Names_of_God, where each one is describing an attribute of Allah. All these indicates that Allah is a supreme being with His own personality and consciousness. So, to be compatible with Islam, any idea about God or Allah has to conform all these points.
Now come to "Pantheism". We see several definition of pantheism. The most common and simple one is-
"Everything is God".
Here "everything" is normally interpreted as "the nature" or "the universe" which we are within and experiencing. That means the nature or universe is identical to God. This is almost same as the other statement-
"The universe or nature is a manifestation of God".
Note I omitted your plural words here. Another definition which expands the first one (and the one you emphasized here) is-
"Everything that exists- this nature or universe including all matter, energy, space and time or any other existence(if there are any) of other universes or forms or ideas beyond our experience and knowledge, combined in totality- is God".
Now, for the sake of answering, let me mention some other idea similar to pantheism-
Monism: "Everything is one."
Panentheism: "Everything is IN God."(Not to confuse with pantheism, spelling is similar)
Now let's examine whether these are compatible with Islam or not. In my understanding, pantheism is the same idea as monism. Pantheism merely mentions the word "God" and that's all. They do not mention any personality or consciousness of God or "The Unity". And sometimes this attributes is clearly denied. And even if we account the other descriptions where the denial is not clear, people of this doctrine also held these ideas (i.e. pantheism or monism) sufficient to describe God. So these can be viewed as indirect denial and thus Monism/Pantheism are clearly not compatible with the view of Islam as I have already mentioned.
Then come to "Panentheism". While quite similar to pantheism regarding the totality saying "everything is in God", it tends to accept the "personality or consciousness" of God (If not, it is also incompatible with Islam). But if it accepts all Islamic attributes of God, then will it become compatible with Islam? Here we need some further discussion about philosophical classification of existence- Physical and Mental. The physical existence is that of the matter and energy. In contrary, mental existence is existence of thoughts or information. And any of these two kind of existence can dissolve into another. For example, our brains or the memory or processors of computers are physical matters but they contain thoughts or information. On the other hand, think of a computer game. It has it's own materials, energy and physical rules, which may or may not resemble our own physical world. But all these things are nothing but data or information within the computer. Those material/energy/physics are "virtual" to us. There are even two topics in philosophy regrading this-
Now, let's go back to Panentheism which states- "Everything is in God". If this existence of everything is to be taken physically, that we, along with our physical universe or nature, is in God or part of God. Then it can be said- God or some part of Him is confined within the space, time and laws of our universe. But this is incompatible with the views of Islam. According to agreed upon view of Islam, Allah is not confined within our space, time and laws. Rather He is the creator of all these. And again, if it is said that, we and our universe is physically part of God, then it would mean that He did not actually created us from nothing. It would seem everything is a kind of transformation of parts of Himself. But Allah has stated that- He has created everything just by His will. Even He has created the events we perform or experience. So, materialistic Panentheism go with Islam.
And if we go the other way- the everything of this universe or reality is actually thought/information or will, and that is in God or mind/knowledge/will(or may be something beyond our knowledge/imagination) of God, then it becomes somewhat compatible with views of Islam. And in my understanding, this is the idea of "Wahdat al-Wojood" mentioned by different mystic people within Islam. I used "somewhat" because this idea is very difficult to grasp and express. Perhaps, my wording are not accurate and someone may go wrong way from silly confusions. Again, as I have mentioned, this is merely compatible with the Islamic view of God, not the Islamic view itself. So, someone can have a different view still compatible with Islam. And someone can even just skip all these idea and maintain the basic Islamic beliefs. And to get closer with the definition of Pantheism you mentioned, we may say, this universe/nature and everything within it manifest different attributes of God.
Hope we have had enough discussion regarding the topic. If anyone find any inconsistency in this answer, please mention it in comment.