When gas flows inside a vertical tube in which a thin liquid film runs down its wall, interfacial shear stress occurs at the gas-liquid interface. This stress is caused by the imperfectly smooth surface of the film running down. For intensification of heat transfer in heat exchangers where the vapour condenses, it is necessary to pay attention not only to the thickness of the liquid film on heat exchange surface, but also the character of the liquid film. This paper describes the influence of a gas flow velocity on a liquid film flow. The gas velocity effect is examined for a constant thickness of the liquid film. When the velocity of the gaseous medium changes, it is necessary to increase or decrease the liquid flow in order to keep the film thickness constant. The effect of shear stress is described for three different inner tube diameters (15.0, 20.0, and 25.0 mm) and for three different theoretical film thicknesses derived from the Nusselt criterion. The results are compared with theoretical, analytical relationships. In all the three tube diameters tested, the influence of the gas velocity is the most significant at low speeds, where the deviation from the theoretical course is the greatest. As the tube diameter decreases, the shear stress effect increases. At higher speeds of the gas and liquid film flow, pulsations start to occur, the film flow stops increasing, and the trend follows the theoretical, analytical relationships published in the professional literature.