The adverse impact of carbon dioxide and air on the thermal performance of condensers used in multistage flash ( MSF) distillation plants is addressed. For normal operating conditions, the inlet gas flow rates are not sufficient to affect the thermal performance significantly. The heat duty for a typical condenser design is then determined for a broad range of entering noncondensable gas flow rates and vent flow rates. When the inlet steam/C02mass ratio falls below about 250, the thermal performance is strongly affected by the noncondensable gases. When the vent flow rate approaches the minimum vapor/gas ratio based on the coolant temperature, a sharp drop in thermal performance also occurs. However, the thermal performance is not sensitive to the vent flow rate when it is about twice this minimum value.