We develop the potential theory of multicomponent adsorption close to a dew point. The approach is based on an asymptotic adsorption equation (AAE) which is valid in a vicinity of the dew point. By this equation the thickness of the liquid film is expressed through thermodynamic characteristics of the bulk phase. The AAE makes it possible to study adsorption in the regions of both the normal and the retrograde condensation. A simple correlation of the Kelvin radius for capillary condensation and the thickness of the adsorbed film is established. Numerical testing shows good agreement between the AAE and the direct calculations, even if the mixture is not close to a dew point.