Abstract: In many developing countries, the construction of transmission lines often lags behind commissioning of new power plants. This results in insufficient transmission line capacity to evacuate generated power to distant load centres. This study aims to address the problem of overcapacity of installed generation relative to transmission capacity and demand, arising from the misalignment of investments in the respective segments. The study proposes coordinated Generation and Transmission Expansion Planning (GTEP) models that consider surplus generation capacity while minimizing investment and operating costs to unlock suppressed customer demand. We develop a multistage stochastic optimization model for GTEP, as well as isolated Generation Expansion Planning (GEP) and Transmission Expansion Planning (TEP) models, and then compare their performance. The models were developed and simulated using PLEXOS Software by Energy Exemplar. The results show that the combined GTEP model provides an optimal expansion plan that utilizes surplus generation capacity and reduces suppressed demand by prioritizing transmission network reinforcements and then investments in generation. The TEP model mitigates overcapacity and unserved energy in some areas, but if not aligned with generation planning, there will be eventual power supply shortages as demand exceeds generation over time.