An experimental investigation on the role of noncondensable gases during condensation of steam inside horizontal/slightly inclined tubes is presented. In a condenser of a thermal desalination unit, the noncondensable gases flowing within the gas phase cause reduction of performance and efficiency. Many researchers have investigated in-tube condensation for horizontal heat exchangers, but very few works have been performed to study the condensation in slightly inclined tubes with noncondensable gases. Inclination of tubes may be requested to achieve suitable and reliable draining. Experiments were conducted in the following conditions: tube internal diameter 12.6 mm, 20 mm and 26.8 mm; tube indination 7 degrees; inlet noncondensable gas mass fraction omega(m)=5%-42%; inlet mixture Reynolds number Re-m,Re-in= 5000-20,000; local noncondensable gas mass fraction omega(m), = 5%-60%; local mixture Reynolds number Re-m=500-20,000; saturated steam at atmospheric pressure, gravity controlled with stratified flow regime, in which the condensate is collected mainly in the bottom part of the tube due to gravity and it is drawn out by its own momentum. A correlation of mixed gas heat transfer coefficient along a slightly inclined tube, in a gravity controlled flow regime, has been developed, showing a good agreement with experimental results. (C) 2012 Elsevier B.V. All rights reserved.