Approximately 40 percent of the nation’s primary energy is consumed in steam generation for power production and industrial process heating, of which 72 percent ultimately is rejected to the environment through condensation. These liquid-vapor phase-change heat transfer processes are often the bottleneck for system efficiency and intensity. To address such limitations, engineers are developing increasingly sophisticated designs for heat transfer equipment. However, due to the complex nature of two-phase transport and the great range of scales, exact analytical models are only feasible for the simplest flows. There is thus a need for robust multiscale simulation tools to analyze and optimize phase-change heat transfer processes in complex configurations.<br /> This presentation will introduce the underlying mechanics of liquid-vapor phase-change and the formulation of an open-source parallelized extensible solver released by our research group. A case study will be presented for analyzing a flow boiling thermosyphon as employed in a waste-heat-activated absorption refrigeration system. Here, simulation findings informed a refined engineering flow boiling model that captures wake heat-transfer enhancement effects. Ongoing work in dropwise condensation will be presented. This transport mode can achieve heat fluxes 10 times those of filmwise condensation and has attracted interest for applications in power plant condensers. A new multiscale formulation is presented that captures average behavior of numerous microscale droplets and directly resolves large-scale interface dynamics. An outlook will be presented for the future of phase-change simulation and its role in enabling energy system advances.