I develop physically-based renderers, the output of which is easily mistaken for photographs. I don't presume myself to be a god of course, but it does make me think... what if you piece together enough specialists for this that and the other aspect of a simulation, and run them on a freakishly powerful computer, with a sort of genetic algorithm simulating virtual beings...

There's a great short story by Greg Egan about this kind of thing, and even with this "spoiler" context it's still very much worth reading: http://ttapress.com/CrystalNights.pdf

Greg Egan - Permutation City. Very nice book.

I didn't like one aspect of the book, when he was trying to show that what simulated people feel doesn't depend on the way simulation runs, he was mainly focusing on computing later timeframes of the simulation before earlier ones, which is not possible.

For the simulated entities? Yes you are - you have absolute power over the simulation.

Not just any simulation, but only a simulation with thinking creatures would imply that god exists. And not in a sense that our universe is created by a god, but in sense that creator of the simulation would be the god of the simulated universe.

That is certainly true for various existing definitions of "god".

And that god could be an evil monopolist?

Oh, plenty of them. Being a god doesn't imply being good nor interested too much in the entity you are a god to.