Of course, there is nothing inherently un-Islamic about capitalism, if by capitalism, we mean competitive free markets in which only the fittest enterprises survive and thrive. That model does not exist in the real world, however, least of all in the West.
The largest shareholder of General Motors and Citibank is the United States government; all successful corporations in the US are utter failures that have avoided collapse by means of massive subsidies, government contracts, and financial bailouts.
That is not capitalism; that is extreme interventionism; that is the owners of wealth mobilizing the mechanisms of the state to guarantee their investments with public funds, precisely to protect them against free market principles. This is very far from capitalism.
Capitalism is based on the legitimate demands of the market, not the fabrication of demands. It is based on the idea that consumers make rational decisions to choose goods which best suit their needs; it is not based on misleading and confusing consumers and cultivating irrationality. This is far from capitalism.
Capitalist theory requires only moderate disparity of wealth in society so that money will be in continuous circulation through sale and purchase of goods. The kind of atrocious income disparity that has developed under the radical distortion of capitalism propagated by the West, creates a perpetual economic crisis, because the wealth of the super-rich never really enters into the active economy, and the lower and middle classes do not have disposable income to contribute to economic growth. Again, this is far from capitalism.
We have to deal with the reality of power dynamics in our society, not theories about economic or political models that only exist on paper. We are, and always have been, and always will be, in a struggle. The rich and powerful have their tools and weapons, and we have ours. They are aware of theirs, and use them, we, however, have remained largely ignorant of ours, and have mostly failed to use the tools and weapons we have.