A joint motion of thin liquid film and dry gas in a microchannel is investigated numerically at different values of initial concentration of the liquid vapor in the gas phase, taking into account the evaporation process. Major factors affecting the temperature distribution in the liquid and gas phases are as follows: transfer of heat by liquid and gas flows, heat loses due to evaporation, diffusion and heat transfer. The velocity and temperature fields in the liquid and gas phases, as well as the vapor concentration in the gas, were calculated. It has been established that in the zone of entry of flows into the channel near the interface, thermal and concentration boundary layers are formed, whose properties differ from the classical ones. Comparisons of the numerical results for the case of the dry gas and for the case of equilibrium concentration of vapor in the gas have been carried out. It is shown that use of dry gas enhances the heat dissipation from the heater. It is found out that not only intense evaporation occurs near the heating areas, but also in both cases vapor condensation takes place below the heater in streamwise direction.