Natural convection laminar film condensation on a vertical flat plate under the thermal boundary condition T_w* = const, is studied to demonstrate the advantages of an asymptotic approach to rather complex two−phase flow and heat transfer problems. After introducing two perturbation parameters, one for subcooling the liquid (∈) and another one for superheating the vapour (∈^), a regular perturbation solution with only one solution parameter is obtained. The effects of variable properties are included asymptotically. Non−asymptotic solutions are far less general than asymptotic ones since they depend on a large number of solution parameters.