A theory has been devised for predicting condensation heat transfer in the presence of a noncondensable gas. The analysis is based on the conservation laws alone and does not utilize empirical data. It is shown that the presence of a very small amount of noncondensable gas in the bulk of the vapor can cause a large buildup of the noncondensable at the liquid-vapor interface. A consequence of this buildup is that the partial pressure of the vapor at the interface is reduced. This, in turn, lowers the temperature at which the vapor condenses and diminishes the effective thermal driving force. Heat-transfer reductions of well over fifty percent may be brought about by the presence of the noncondensable. The predictions of the analysis are compared with condensation heat-transfer measurements for steam with air as noncondensable. The agreement between theory and experiment is satisfactory.