Playing in-game makes my mac desktop shutdown [CRASH]

*For reference: I play on a 2009 iMac, running OS X Yosemite (version 10.10.5)

My issue is that every time I'm playing in-game, my client either freezes (making me restart my entire computer) or shuts down/crashes (making me start up my computer). I've also found out that when I have to restart/startup my computer again, it doesn't disconnect my from my client; I've gotten AFK reports on me, because it takes ~8-10 minutes for me to get back in the game.

Similarly, this also happens when I play on my macbook pro 2013 laptop. I don't remember what it OS X it runs on, but either way, the same problem happens.

However, I've noticed that on my iMac it's more prone to force shutdowns, while my laptop is more prone to freezes. I don't know if the observation is useful, but I thought I'd throw it in there.