An experimental and analytical investigation has been conducted to determine the effects of the presence of noncondensable gases on in-tube steam condensation under forced convection conditions. The local mixture Nusselt numbers were correlated in terms of the local mixture Reynolds number, mixture Jakob number, and the gas mass fractions or the mixture Schmidt number. Correlations including the mixture Schmidt number did better in representing the condensation process in the presence of one noncondensable gas. The experiments covered steam/gas mixture inlet Reynolds numbers from about 6000 to 26 000, inlet helium mass fraction range from 0.022 up to 0.20, inlet air mass Fraction range from 0.045 up to 0.20, and mixture inlet temperature of 100 degrees C and 130 degrees C (corresponding to a pressure range from 114 to 603 kPa). The ratios of the condensate film thermal resistance to the thermal resistance of the steam/noncondensable gas mixture at the same bulk temperature were calculated. In general, the condensate film thermal resistance was found to be significant for turbulent gas mixture conditions (Re > 6000) and relatively low gas mass fraction (W < 0.2). A diffusion-based, simplified boundary layer model has been developed. The steady-state radial diffusion equations of the mixture components were solved in order to obtain the steam mass flux at the interface between the condensate film and the mixture. The effect of the axial flow was included in terms of an effective boundary layer thickness which decreases with mixture Reynolds number. The model included the effect of more than one noncondensable gas on steam condensation. The model-predicted mixture heat transfer coefficients were favorably compared to the experimental results.