Using pure hydrogen or hydrogen/natural gas blends in gas-fired power plants poses a lower carbon footprint. This paper evaluates replacing natural gas with hydrogen/natural gas blends in existing power plants. Five gas-fired power plants were evaluated. For a comprehensive evaluation, these power plants were selected in different countries: Australia, Denmark, Germany, UK, and US. Energy, exergy, and economic analysis were applied to depict the implications of the various blends. The simulation was performed using EBSILON Professional software. The results demonstrate the following advantages: lower carbon emissions and higher energy and exergy efficiencies keeping the net power output constant. However, NOx emissions and the volumetric flowrate of the fuel (hydrogen/natural gas blends) increased drastically, i.e., by nearly 16% with the introduction of 20 vol%H2. This affects to design parameters of the gas-fired power plants. The introduction of hydrogen into natural gas affects the levelized cost of fuel. However, it depends on the price disparity between hydrogen and natural gas. The main technological and economic constraints of using hydrogen/natural gas blends in existing gas-fired power plants are assessed in this paper as well.