The non-condensable gas model is built upon the two-fluid, liquid/vapor phase change model and includes the effects of a non-condensable gas present in the vapor space. The new model is designed to work only with the two-fluid phase change model because the spatial distribution of vapor and gas components is needed to predict the phase change behavior. By contrast, the one-fluid phase change model assumes spatially uniform pressure and temperature throughout the gas phase. The assumption of a uniform gas/vapor concentration in a two-component gas would rarely be valid. This model is the basis of work completed to simulate the ullage space of cryogenic tanks2, but it is applicable to any two-component gas problem.