An analytical study was conducted to characterize the local condensation heat transfer coefficient of a vapor in the presence of a noncondensable gas, where the gas mixture is flowing downward inside a vertical tube. The two-phase heat transfer was analyzed using an annular flow pattern with a liquid film at the tube wall and a turbulent gas/vapor core. The liquid phase heat transfer was modeled as heat conduction across a falling film. The gas/vapor core was modeled using the analogy between heat and mass transfer. Emphasis was placed on including the effects of developing flow, condensate film roughness, and property variation in the gas phase. The predictions of the model were compared to the experimentally obtained data and reasonably good agreement was found. The results obtained show that for the same mass fraction of noncondensable gas, compared with air, hydrogen and helium have a more inhibiting effect on the heat transfer in that order, but for the same molar ratio, (a) air was found to be more inhibiting, and (b) the heat transfer characteristics of hydrogen/steam and helium/steam mixtures are nearly identical. The results also show that the effects of developing flow are negligible when the inlet flow is at high turbulent Reynolds numbers (Re > 10000). Also, the results show that the film roughness effects are negligible for gas mixtures with low Schmidt numbers (Sc <1.0).