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The work is devoted to the study of the gravitational motion of a liquid film under evaporation conditions into a flow of neutral gas as applied to film machines with a plane-parallel nozzle. The aim of the work is to develop a mathematical model of such a process and establish its laws. The model is based on the physical concepts of a liquid film flowing down a flat surface heated from outside under the assumption that the film flows in isothermal conditions under steady-state laminar mode without wave formation and in the absence of friction between the gas and the film. The mathematical description of the film flow down process in these conditions includes the equation of motion and the continuity equation for the liquid film, which are supplemented by the equations of mass transfer, the material balance of the gas phase in the evaporated liquid, the relative content of the vaporized substance in the gas phase and the equation expressing the Dalton's law. As a result of solving this system of equations, dependencies are obtained that make it possible, at known values of the mass transfer coefficient in the gas phase, to carry out calculations and simulate the hydrodynamics of the liquid film flow under conditions of evaporation into a flow of neutral gas. For the water-to-air system, regularities were established in which the film thickness and speed of movement along the surface height were varied for different types of film interaction with the air flow: forward flow, backflow, and cross-flow, as well as with different hydrodynamic and temperature conditions of its flow down under cross interactions conditions of the flows. It has been shown that in all cases a decrease in the thickness and speed of movement of the film is observed, with the largest decrease occurring during cross-interaction, which is associated with an intense removal of the resulting vapor from the film surface. The developed mathematical model can be used to evaluate the operating modes of film machines with a plane-parallel nozzle.