A mathematical model is proposed to design condensers for vapor mixtures containing a non-condensable gas. The Colburn and Drew (1937) film model is used, assuming the liquid flux to be completely mixed, but taking into account the interactions due to diffusion in the gaseous mixture. Some computational examples are given to show how the present model compares with the previous ones for the condensation of water-methanol in the presence of air in a single tube condenser, and how the condensation of water-ethanol mixtures in the presence of carbon dioxide in a shell-and-tube condenser evolves at different operating conditions.