Maybe they forgot to put a new thermostat back in. The purpose of the thermostat isn't to keep your car from overheating, it's there to hold the water in the motor until it heats up to operating temperature. If your temp gauge is working properly, the only thing that can cause this is a bad/wrong thermostat. I bet your gauge and sensors are fine because your heater is blowing luke-warm air... So take it back to the idiots that did the job and (nicely) ask them to fix it.