I have a 2013 dodge dart and it keeps on overheating.

First thing to do is find another mechanic. I'd suspect a blown head gasket since it's overheating at idle and while driving. The coolant should be tested for exhaust gasses. If exhaust gasses are present in the coolant, the head gasket is definitely blown. Other possibilities are a bad thermostat, and a blockage in front of the radiator. Since you have good heat the coolant is circulating. That rules out the water pump, air in the cooling system, and a blocked cooling system. There's a bypass on the thermostat to allow coolant to circulate just through the heater core when the engine is cold and the thermostat is closed. But if the thermostat is stuck closed you will have plenty of heat but the engine will overheat because the coolant is not circulating through the engine! Hope that helps! Jim