I've been reading about this topic in a book recently and it states that when steam (water vapor) condenses on a cold surface in the presence of air, the increase in air concentration at the condensation interface causes a reduction in the steam pressure and hence, temperature. I am wondering why this is the case? Why does the overall system pressure not just increase? For example, in a condenser operating with non-condensables (such as air), I understand that the partial pressure of air will increase at the condensation interface due to the increase in concentration there but would that increase in partial pressure not just increase the overall condenser pressure instead of depressing the steam pressure & temperature? Please feel free to show me the error of my ways.