The Heat Monitor was a device that helped the planet's inhabitants monitor their thermal energy level. In some ways, the Heat Monitor replaced the Oxygen Meter from the first two Dead Space games. Only this time, Oxygen was plentiful, but an environmental hazard monitoring device was still being used.
The only instance of needing to keep an eye on your body temperature was Chapter 8: Off The Grid, just after the shuttle crashed onto the planet's surface. Isaac's (And Carver's in Co-Op) helmet began to malfunction and left his head open to the hostile environment. Once the players reached the Suit Kiosk and switched suits in Chapter 9: Onward, the heat monitor was no longer thrown into play. Despite the protection against the climate that the suit offered to the rest of their bodies, Isaac and Carver would die if they are exposed to the cold environment for too long. The monitor tracked their body heat as they wandered around topside. The heat depleted as they are exposed, but their suits would slowly warm them back up once they are out of the elements.