The Generalized Replica Exchange Method (gREM) was applied to simulate a solid-liquid phase transition in a nanoconfined bilayer water system using the monatomic water (mW) model. Merging an optimally designed non-Boltzmann sampling weight with replica exchange, gREM is particularly well suited for the effective simulation of first-order phase transitions characterized by S-bends (“backbending”) in the statistical temperature and a bimodal structure in the canonical probability density function. The effective temperatures of gREM were designed to form unique crossing points with the statistical temperature, thereby facilitating sampling of energy states across the transition region. Statistical Temperature Weighted Histogram Analysis Method (ST-WHAM) was used to reweight gREM simulation results into canonical ensemble averages, including the Helmholtz free energy, internal energy, and heat capacity. The minimized structures of bilayer water systems with varying sizes were obtained through basin-hopping global optimization using the GMIN package, and ice structures composed of pentagons, hexagons and heptagons were observed.