Nissan Almera 1.4?

My Nissan seems to be overheating lately. Took it to the garage. Got the thermostat changed, the water pump...and the sensor, garage also tested car to make sure it was not the head gasket....still overheats in traffic. I was told not to put any water in the plastic bottle on whch was written " coolant". I was told it was for the overflow ? Is this the case ? what else coud be causing the problem ?