Power-to-methane, a component of power-to-gas, is recognized globally as a key technology in the transition to a sustainable energy system. Methanation is the process of synthesizing methane (CH4), the main component of city gas, from hydrogen (H2) and carbon dioxide (CO2). This process typically involves the Sabatier reaction, which uses an inorganic catalyst to react H2 and CO2 to produce CH4; however, it can also be carried out by microorganisms, a process known as biomethanation. Archaean methanogens, particularly hydrogenotrophic methanogens of the genus Methanothermobacter, have been extensively studied in methanation research due to their capacity for mass cultivation. Understanding the methanogenic pathways of these organisms is valuable for enhancing biomethanation processes. Various methods for biomethanation, including in situ and ex situ types, have also been studied, with discussions surrounding the advantages and disadvantages of each. In addition, various reactor designs have been developed to improve the efficiency of the biomethanation reaction.