The performance of convection-condensation heat transfer of mixture gas with water vapor in a vertical tube was studied experimentally. The convection heat transfer between bulk flow and interface and condensation heat transfer on the liquid film were considered. The convection heat transfer is increased by the condensation of the condensable gas (water vapor) existing in the wet flue gas. This paper discusses the effects of the condensation of a small amount of water vapor (10%-30% fraction) on convection heat transfer in a vertical tube. The flows (Re=3000 – 10000) in vertical tube were investigated experimentally. Some interesting results were obtained. The wall temperature was an important factor affecting the condensing rate. The proportion of the sensible and latent changes with the Re number.