Hi, the condensation is from the air conditioning coil. You will find a 3/4" white plastic drain line going from the furnace to an outside location. you need to clear this drain line.
If you cut it with a a saw or somthing you can try to blow it out with compressed air or even try hooking the water hose up to it. Or you can put a fish tape down it. If none of that is possible, go outside where the ac is located an look for the pipe coming out of the wall or ground. That is where it usually gets pluged.