Laminar film condensation from mixtures of a vapor and a lighter noncondensable gas flowing downward along inclined isothermal plates is investigated numerically using the full boundary-layer equations for the liquid film and the mixture. The adverse net body force due to high concentration of the lighter gas near the liquid-mixture interface gives rise to the possibility of boundary-layer separation. A numerical solution was developed based on a finite-control-volume method and it was marched along the plate up to the separation location. Results were obtained for three vapor-gas mixtures covering a wide range of liquid Prandtl numbers, high gas concentration and high liquid subcooling. Comparisons with previous models (which included various simplifying assumptions) showed that agreement is possible only for conditions of low gas concentration and liquid subcooling. Results of the separation distance exhibited interesting trends, which are discussed and explained. It is also demonstrated that lighter gases can have a more inhibiting effect on heat transfer than heavier gases.