Okay, you're computer overheats and turns off? You were disconnected for a certain amount of time, which leaves your team at a disadvantage for however long you were gone. Despite the fact that you came back and claim you carried your team to victory, you still left them at a disadvantage during the time you were gone.
Now, a few other things. First, you do not get punished for only one leave, you get punished when you show a habit of leaving/disconnecting from multiple games. Riot knows that sometimes irl things come up and you need to leave for whatever reason, so Leaverbuster does have some built in leniency. So if you got punished for this one disconnect, you have to have some history of disconnecting from previous matches. Keep in mind that it's possible that Leaverbuster keeps track of leaves for quite a while.
Secondly (and I'm not saying that this is necessarily what you are doing), whether or not the reason you are leaving is out of your control, you still have control over queuing yourself up for a match. If something is wrong with your ability to stay connected to a match and you know that there is something wrong, why would you continue to queue up for matches and risk disconnecting? You should do your best to identify the problem and get some help (if needed) to fix the problem before queuing up for any more matches.
For your specific problem, your computer overheated. That sucks, and it sucks that this specific disconnect pushed you over the edge and caused you to receive a punishment. However, while this may not be entirely under your control, you have to understand that this specific dc was not Riot's fault at all. Riot had nothing to do with your computer overheating. You were ore at fault than Riot was for this particular incident.
Also, it can be difficult to determine caused the leave, whether it was a power outage, your computer overheating, or possibly you disconnecting your computer from the internet to make it look like you lost your internet connection. This is one reason why they group all disconnects together and and punish you for leaving despite what the reason may have been.
Now, for you, you're going to have to take the punishment, which is unfortunate, but it is not going to be lifted. However, I would look into figuring out how to stop your computer from overheating during a league match so that this doesn't happen to you again.
Having said all of this, I do believe that to possibly make the Leaverbuster system a bit better is, if the leaver does reconnect to the match and continues playing, to possibly have the leaver's teammates decide whether or not Leaverbuster should mark that particular match as a leave. This would depend on how well the player contributed to the match when he/she came back.