Poland is facing demanding challenges to achieve a sustainable energy mix in the near future. Crucial and tough decisions must be made about the direction of the national energy economy, safety, and environmental impact. Considering the electricity and heating demand forecast, this paper proposes an optimization model based on the Grey Wolf Optimizer meta-heuristic to support the definition of ideal energy mix considering the investment and operational costs. The proposed methodology uses the present energy mix in Poland (the most recent values are from 2017) to calibrate the model implemented in the EnergyPLAN tool. Afterwards, EnergyPLAN relates to an optimization process allowing the identification of the most convenient energy mix in 2040 in Poland. The values obtained are compared with those proposed by Polish public entities showing advantage regarding the global costs of the project nevertheless respecting the same levels of CO₂ and the energy import and export balance. The expected savings can achieve 1.3 billion euros a year and more than 8 million tonnes of CO₂ emission reduction. Sensitivity analysis considering the decrease of the global cost of renewables-based sources is also presented.