Maybe you have the necessary info, but simply refuse to accept it.
Just because your machine runs one game without overheating, does not mean it will run another without overheating.
GalCiv2 puts a far greater strain on the CPU (and GPU if you are zoomed in) than most other games. And that generates more heat.
You are running an Intel based game on a Mac on Boot Camp, besides. (which is sort of inside outside in)
So if you won't consider both those factors, then yes, give up on running GalCiv2.
But, if you do consider them, then take the steps to verify it.
Do you have a program that can show CPU and GPU temps during game play? Because that might just show if overheating is really an issue.
But, if you just say - this can not be, because a different game runs fine...
well, figure it out for yourself - with your more than limted information base.