In this thesis, a detailed model of the ECC-SMART small-modular supercritical-water reactor has been made using the OpenMC Monte Carlo code. For this purpose, three models have been developed with different levels of fidelity: a 2D infinite lattice pin-cell model, a 3D finite-length fuel assembly model including end structures, and a full-core model. The core design features a 20×20 square lattice of horizontally oriented fuel assemblies, each containing 40 UO2 fuel rods with an 8 mm diameter and 168 cm active length, traversed by coolant flowing horizontally through seven heating stages, from subcooled to supercritical conditions. An offline neutronic/thermal-hydraulic coupling was achieved by integrating the OpenMC model with an existing RELAP/SCDAPSIM model developed within the ECC-SMART framework. The temperature and density distributions from the RELAP/SCDAPSIM model were mapped to the OpenMC model, and the power distribution was iteratively fed back to the thermal-hydraulic model. Convergence was reached within five iterations, yielding a neutron multiplication factor of keff = 1.19338 ± 0.00008, below the target value of 1.22 required for a two-year cycle. The power distribution exhibited as expected a bottom-skewed axial shape due to coolant density variations, with a power peaking factor of PPF=2.006 ± 0.003. High stage peaking factors (SPF > 1.6) were observed in upper and lower stages, due to leakage effects and geometric asymmetries, indicating the need for better power flattening to lower peak temperatures and achieve more uniform burnup. Temperature feedback was assessed by locally perturbing the converged thermal-hydraulic distributions without re-running full coupled iterations, applying fuel temperature variations of ±150 K/±300 K and coolant temperature variations of ±5 K/±10 K. Density updates were calculated using the IAPWS-IF97 standard. The result obtained for the global coolant temperature coefficient is within the typical range of characteristic values for light water reactors. Although not calculated, it is expected from the individual stage coefficients that a global estimate of the fuel temperature reactivity coefficient will be within the order of magnitude for light water reactors as well. The resulting reactivity feedbacks provided insight into the core's safety margins and the sensitivity of reactivity to localized temperature changes. Finally, the change of the moderator-to-fuel ratio was investigated by varying the gap between fuel assemblies from 14 mm to 25 mm. The reference configuration (18 mm gap) operated in an under-moderated regime, yielding a negative reactivity margin during transients. Optimal moderation occurred near a 20 mm gap. Taking advantage of the Python application programming interface (API) available with OpenMC, all the models can easily be customized and allow performing, automatically, a large variety of parametric studies.