Combined-cycle gas turbine (CCGT) power plants play an important role in reducing greenhouse gas emissions while maintaining high thermal efficiency. Accurate thermodynamic models are essential for evaluating plant performance under different operating conditions and supporting improvements in energy resource management. This study presents a Python-based thermodynamic model of an isolated natural gas-fired CCGT power plant, applied to 1027 steady-state operating points recorded over one month of operation. The model integrates energy, exergy, and exergoeconomic analyses based on the Specific Exergy Costing (SPECO) methodology to assess plant operation with and without supplementary firing. The model was validated against the measured steam production, with a mean absolute percentage error of 2.8%. The proposed methodology enables the evaluation of component efficiencies, exergy destruction, cost formation, and the influence of different operating modes on electricity production. The plant achieved median gross first- and second-law efficiencies of 46.6% and 44.6%, respectively (45.1% and 43.2% on a net basis). The combustion chamber exhibited the highest exergy destruction in both operating modes, while the supplementary burner became one of the major sources of irreversibility when supplementary firing was activated. The specific exergy cost of electricity decreased from about 21.0 to 14.2 $/GJ as the electrical output increased from 11.3 to 20.9 MW, with a median of 16.47 $/GJ in the period. The supplementary burner presented the highest relative cost difference (median of 1.06), whereas the combustion chamber had the highest exergy destruction rate and cost rate of exergy destruction. The proposed model provides a flexible and reproducible framework for evaluating CCGT power plants and can support operational assessment and decision-making aimed at improving thermodynamic and economic performance.