I just bought Asus r558UQ and started playing some of my old games Like WATCH-DOGS , Witcher 2 and gta4. i usually play games on mid settings but i dont know why my " Laptop is Heating on Mid & low settings". There is no Glitch on high Settings or on medium settings . But Heating is a major issue for me .
If it is over heating, I would really suggest you install some monitoring software and see what kind of temps you are getting. If they are high, or higher than normal, then I would suggest you open it up and clean it out of all dust and debris and see if that lowers the temps.
Should they still be up there, then you may well want to try replacing the thermal paste. And if that still doesn't do it, look into replacing the fan(s).
NOTE: Do not open it up if the device is still under warranty. If that is the case, contact the manufacturer about a repair/replacement.