An analytical investigation on the role of non-condensable gases (NCG) during condensation of steam outside vertical tubes in a seawater desalination multiple-effect plant was carried out. The heat and mass transfer process for a single tube was described, using a two-phase boundary layer model of Minkowycz (1965), together with an evaporation heat transfer model inside the tube. Heat transfer coefficients and condensate (product) flow rates were obtained as a function of the NCG concentration. Based on this, a picture of the NCG concentration distribution was computed for a tube-bundle of a single effect. The influence of this distribution on the heat transfer coefficient along system tubes was also examined. The model was implemented in an iterative algorithm and a computer code. The total pressure of the steam was 5.72 kPa (corresponding to the saturation temperature of about 35 degreesC). The NCG concentrations were 0.001-2% mass. An increase was found by a factor of 1.4 to 2 of the NCG concentration in the steam flow between the tubes, depending on the inlet value. The corresponding decrease of the overall heat transfer coefficient was up to 10%. The product output of the whole stage displayed a decrease of up to 50%. The lowest value of the NCG concentration that demonstrated any significant effect on the overall coefficient in this work was 0.1%.