Atmospheric entry is a crucial phase in planetary exploration missions. During entry, the vehicle experiences severe heating at hypersonic speeds. To ensure the survival of the payload, this heating needs to be mitigated using thermal protection systems. Ablative shielding materials dissipate the incoming heat largely by surface reactions leading to material decomposition. Accurate simulations of this flow environment are critical for the efficient design of spacecraft. The main difficulties in this analysis are due to strong shock waves and thermochemical nonequilibrium in the flow field, giving rise to chemical reactions and the excitation of the internal energy modes of the fluid particles. Under these conditions, an accurate assessment of the flow field requires detailed models for evaluating the physicochemical properties of the reacting fluid, and its interaction with the heat shield at the vehicle surface. This thesis considered the coupling of a high-fidelity flow solver with an aerothermodynamic library designed specifically for atmospheric entry applications. The flow solver is a Cartesian grid immersed boundary finite volume code capable of performing high-order accurate simulations. The coupling procedure with the external library involved four modules to extend the applicability of the flow solver to atmospheric entry flight regimes. The first one provides an accurate set of thermodynamic properties acquired from a tailored database for relevant species. The second module supplies transport properties through a rigorous calculation respecting kinetic theory as opposed to simplified models. The third module deals with the finite-rate chemical reactions occurring in the flow. The last module enables the consideration of catalytic and ablative surface reactions as boundary conditions for the flow solver. Thermal nonequilibrium is implemented to consider two temperatures for the conservation of translational-rotational and the vibrational-electronic energies. Gas-surface interactions are implemented for the first time in a conservative immersed interface method. Various test cases have been simulated for the verification and validation of each of these implementations. Good agreement with reference results are obtained. The increase in the computational cost is justified by the significant improvements in the results and the wide range of conditions made accessible for investigation. A novel framework is established, with which flow simulations that are state-of-the-art both in terms of numerical accuracy and fluid physicochemistry can be performed.