You are probably noticing heat from the CPUs & GPU, not the battery. It's entirely normal for games to heat the phone anywhere from warm to rather hot. The high performance 8 core Snapdragon can generate significant heat under heavy load; actually far more heat than can ever be removed from a phone-sized device via passive cooling. There is thermal protection which cuts the performance when it reaches max safe temperature (around 50C). Some of that heat may end up in the battery as well, but it's probably not the significant source unless it is charging at the time.