Gases condense when they're cooled because the molecules inside the gas are becoming slower as they get closer to each other. They have less energy to move when they're being cooled down.
Imagine a container of a gas, they're moving around. Think of it this way, when it's really hot, you move and squirm around. when it's cool, you're more relax. Gases are faster when the temperature is hot. The cooler it gets, the slower the gas becomes, so it ends up condensing.