Conjugate heat transfer in supercritical hydrocarbon fuels within microchannels is strongly influenced by sharp thermophysical property variations and chemical reactions, posing significant challenges for accurate numerical prediction. To address this, a high-fidelity solver is developed within the OpenFOAM framework, incorporating detailed reaction mechanisms and demonstrating robust stability under steady supercritical conditions. In particular, to mitigate numerical oscillations and accuracy loss in the pseudo-critical region, a high-order variable-property transport model, based on an eight-segment, seventh-order polynomial formulation, is introduced and integrated in the solver. This model is tightly coupled with the Peng–Robinson equation of state and a simplified cracking mechanism, enhancing both stability and predictive capability for highly nonlinear supercritical reacting flows. The proposed approach is applied to compare the coupled thermal–hydraulic–chemical behavior of a baseline straight channel and a trapezoidal-ribbed configuration under supercritical pressure. Unlike conventional fully distributed rib arrangements, the proposed design achieves heat transfer enhancement using a limited number of rib elements. The ribs promote elevated turbulent kinetic energy in their wake and intensify near-wall mixing. As a result, peak wall temperature is reduced by 30–40 K under identical conditions, effectively suppressing localized hot spots. Although improved cooling slightly decreases the cracking conversion rate, the design markedly lowers the risk of fuel coking by eliminating high-temperature regions, thereby enhancing overall thermal management. The performance evaluation criterion (PEC) remains close to or slightly above unity across different mass flow rates, indicating a modest but meaningful thermo-hydraulic benefit and practical engineering potential.