high pressures and changing states

how does increasing the pressure of a gas cause it to change its state? When we bring an object to high grounds, a liquid (water) has a lower boiling point as it has a low pressure. What about the gas itself? If i heat up the gas or decrease the volume of gas it would increase the pressure, so would it be harder to condense? The temperature variable is straight forward but not the volume one.

Thanks for the help!