A gas may be liquefied by cooling below its boiling point (bP) at normal pressure.
However, some gases such as oxygen, and nitrogen have very low bP, it’s thus not practical to do so.
Practically the reduction of the r.m.s. speed of the gas molecules (=cooling the gas) is adopted.
When the r.m.s. speed is reduced until the molecules come closer and captured each other by intermolecular attraction, the cooled gas will condense.