In the present work vapor back flow phenomena in single-pass, multiple-row, cross-flow air-cooled condensers are investigated. The condensation in this kind of heat exchangers is studied with the help of a mathematical model, which considers the effect of changes in the saturation temperature inside the tubes and changes in the air velocity in the direction transversal to the coolant flow. The model equations are solved with a new algorithm. The vapor distribution, the pressure drop between the inlet and outlet plena and the global effectiveness are determined under different working conditions.