The growth in worldwide energy demand currently faces the difficulty of installing new power generation facilities due to limited funding and the strengthening of environmental regulations. Owing to the increasing role of natural gas combined cycles (NGCCs) in the power generation infrastructure, it is important to assess the energy and economic feasibility of upgrading their capacity to cope with future energy requirements. This study aims to investigate a new concept for upgrading existing NGCCs by integrating an additional unit based on gas turbine. The additional unit allows the production of steam to be injected into the NGCC combustor (Option A) or syngas from methane steam reforming to be fed into the existing power plant (Option B) or even into the additional gas turbine (Option C). The power output augmentation arises from the capacity of the additional gas turbine and the increase in power production of both gas cycle and bottoming steam section of the NGCC. A preliminary sensitivity analysis assesses the influence of operating conditions of the additional unit on the energy and economic performances of marginal power production. Considering commercial gas turbines, further investigations address the design of upgrading options and the comparison of their techno-economic performances referred to the additional or the overall power productions. Finally, focusing on Option A, the part-load operation of the additional gas turbine is examined to evaluate penalties on marginal efficiency and marginal cost of electricity. Simulation results revealed that for a power augmentation lower than 50%, Option A provides the best economic performance combined with the highest operational simplicity, while Option C is the preferred technical solution for a greater increase in plant capacity. (C) 2019 Elsevier Ltd. All rights reserved.