The heat and mass transfer when a fluid evaporates from the hot wall, and condenses on the cold wall of a rectangular cavity in the presence of large concentrations of a noncondensable gas, are investigated experimentally and theoretically. A Mach-Zehnder interferometer and a thermocouple measure the concentration and temperature distributions and the heat and mass transfer coefficients. Two dimensional, coupled, nonlinear partial differential equations are formulated and solved numerically by an alternating direction implicit finite-difference method, providing internal flow configurations and heat and mass transfer. The results are useful in such practical systems as partial-pressure distillation and solar desalination.