"The purpose of this research was to perform scaled experiments and simulations to validate computational fluid dynamics (CFD) and empirical models of condensation heat transfer (CHT) for the passive containment cooling system (PCCS) of Small Modular Reactors (SMRs). SMRs are the futuristic candidates for clean, economic, and safe energy generation; however, reactor licensing requires safety system evaluations, such as PCCS. The knowledge in the reviewed relevant literature showed a gap in experimental data for scaling SMR's safety systems and validating computational models. The previously available test data were inconsistent due to unscaled geometric and varying physics conditions. These inconsistencies lead to inadequate test data benchmarking. This study developed three scaled (different diameters) test sections with annular cooling for scale testing and analysis to fill this research gap. First, tests were performed for pure steam and steam with non-condensable gases (NCGs), like nitrogen and helium, at different mass fractions, inlet mass flow rates, and pressure ranges. Second, detailed CFD simulations and validations were performed using STAR-CCM+ software with scaled geometries and experimental parameters (e.g., flow rate, pressure, and steam-NCG mixtures), thus mimicking reactor accident cases. The multi-component gases, multiphase mixtures, and fluid film condensation models were applied, verified, and optimized in the CFD simulations with associated turbulence models. Third, the physics-based and data-driven condensation models and empirical correlations were assessed to quantify the scaling distortions. Finally, the experiments, simulations, and modeling results were evaluated for critical insights into the physics conditions, scaling effects, and multi-component gas mixture parameters. This study supported improvements to nuclear reactor safety systems' modeling capabilities irrespective of size (small or big), and findings were equally applicable to other non-nuclear energy applications"--Abstract, page iii.