There are two significant effects of inerts on the condensation of vapors.  The first is the effect on the temperature-composition profile which determines the fraction condensed as a function of temperature.  As material condenses the vapor phase composition becomes richer in air and it becomes more difficult to condense the remaining methanol.  The second effect is to add a mass transfer resistance as the vapor near the liquid film is depleted of the condensing component, resulting in a radial composition profile.  Chapter 16 of “Process Heat Transfer” (Hewitt, Shires & Bott) provides a good treatment of condensation in the presence of an inert gas.  If done correctly the amount of excess surface can be minimized.