The Heat Monitor is a device that helps the planet's inhabitants monitor their thermal energy level. In some ways, the Heat Monitor replaces the Oxygen Meter from the first two Dead Space games; only this time, Oxygen is plentiful but an environmental hazard monitoring device is still being used.
The only instance of needing to keep an eye on your body temperature is Chapter 8: Off The Grid, just after the shuttle crashes onto the planet surface. Isaac's (and Carver's in co-op) helmet begins to malfunction and leaves his head open to the hostile environment. Once players reach the suit kiosk and switch suits in Chapter 9: Onward, the heat monitor is no longer thrown into play. Despite the protection against the climate the suit offers the rest of their bodies, Isaac and Carver will die if exposed to the cold environment for too long. The monitor tracks their body heat as they wander around topside. Heat depletes as they're exposed, but their suits will slowly warm them back up once they are out of the elements.