The new AP600 reactor designed by Westinghouse uses a passive safety system relying on heat removal by condensation to keep containment within the design limits of pressure and temperature. Even though some research has been done so far in this regard, there were still some uncertainties concerning the behavior of the system under postulated accident conditions. In this paper, steam condensation onto the internal surfaces of AP600 containment walls has been investigated in two scaled vessels with similar aspect ratios to the actual AP600. The heat transfer degradation in the presence of noncondensable gas has been analyzed for different noncondensable mixtures of air and helium (hydrogen simulant). Molar fractions of noncondensables/steam ranged from (0.4-4.0) and helium concentrations in the noncondensable mixture were 0-50\% by volume. In addition, the effects of bulk temperatures, mass fraction of noncondensable/steam, cold wall surface temperature, pressure, noncondensable composition and inclination of the condensing surface were studied. It was found that the heat transfer coefficients ranged from (50 - 800$\frac{J}{sKm^2}$) with the highest for high wall temperatures at high pressure and low noncondensable molar fractions. The effect of a light gas (helium) in the noncondensable mixture were found to be negligible for concentrations less than 40 molar percent but could result in stratification at higher concentrations. The complete study gives a large and relatively complete data base on condensation within a scaled AP600 containment structure, providing an invaluable set of data against which to validate models. In addition, specific areas requiring further investigation are summarized.