Rapidly disseminating renewable generation and worldwide transition into decarbonized power plants has triggered drastic transformations in the types and operations of new generation power systems within present electricity grids. Considering the thermal energy production sector of a country like Angola where thermal power generation still dominates, the continuous decommissioning of these units should be examined scientifically to make sure the system stability and reliability. The current study investigates the impact of plant decommissioning in the thermal power sector on power system stability and decarbonization in Huíla Province as a case study. With PSS®E, a simulation-based analysis is designed under various scenarios of the integration of renewable energy when thermal generation is gradually reduced. Among various stability indices that should be taken into account in analyzing the picture: voltage stability, system losses, and frequency response dynamics. There are three operating scenarios in question: (i) the thermal dominated system, (ii) the hybrid system with moderate renewable penetration and (iii) the high-renewable penetration system supported by decentralized generation. The results demonstrate that high renewable energy penetration is statistically associated with a significant improvement in both system efficiency and reduced greenhouse gas emissions, up to 70% for high renewable energy scenario. However, there are problems related to low inertia and voltage stability of the systems especially at high displacement of synchronous generation. The combination of phased decommissioning, adequate grid reinforcement and planned renewables integration is essential in achieving environmental sustainability and operational security. The results of this study will be useful to energy planners and system operators that aim to formulate coherent transitions pathways for power generation systems of the future like that in Angola.