Thermoelectric generator (TEG) effectiveness in boosting hybridized, compressed natural gas (CNG) 3.0 L engines is demonstrated using a model-based development approach. Measured data from the corrugated fin-type TEG under different gas temperatures and mass flow rates are used for validating the model. The accurate TEG model can reproduce measured pressure loss, heat transfer, and thermal performance characteristics. Next, the model is integrated into the spark ignited CNG engine. Predicted engine performances are well-calibrated with measured data from the twin-turbocharged, mass-production engine used in light-duty delivery trucks. The engine model is validated with measured data for 35 conditions under the JE05 cycle (800–2800 RPM, 2.6–102 kW). The results show that the engine brake thermal efficiency (BTE) is improved by 0.56% using a 7 × 9 TEG module arrangement. A 9 × 10 arrangement can enhance the BTE to 0.8%. Effective electrical power is generated up to 1.168 kW from the TEG, depending on JE05 operating regions, without significant power loss.