Abstract: To optimize the upstream syngas production during the Fischer–Tropsch synthesis of sustainable aviation fuel, a system model was developed for a biogas-assisted solid oxide electrolysis cell （SOEC） using a non-upgraded biogas simulant mainly composed of CH₄ and CO₂ as the anode feed. The model included calculations of single-cell voltages and electrochemical reaction rates, anodic equilibrium conversion, heat exchange, and separation processes, and was evaluated at multiple levels using the I–V and EIS data together with the operating conditions reported in the literature. Within the current-density range of 2500–4500 A·m⁻², the mean absolute deviation of the predicted cell voltage was 0.0095 V. Increasing the temperature from 500 to 700 ℃ resulted in an increase in the CH₄ conversion from 73.8% to 99.5% and the Fischer–Tropsch stoichiometric ratio for CO₂-containing syngas （R_FT） from 0.916 to 0.999, accompanied by a decrease in the specific overall energy consumption of target syngas from 2.66 to 0.60 kWh·Nm⁻³ under the investigated conditions. Two-dimensional analysis showed that CH₄conversion, syngas-composition matching, and low energy consumption can be balanced at 600–700 ℃ and anode H₂O/CH₄ratios of 0.5–1.5, and within the coupled CH₄/CO₂–current-density region of 2.6–3.3 and 3000–4300 A·m⁻². Under the representative operating condition, the energy efficiency of the upstream syngas production was as high as 86.9% with a heat-recovery ratio of 45.0%. The results can provide a reference for operating-condition screening in Fischer–Tropsch syngas production using non-upgraded biogas-assisted SOECs.