Methanation of CO/CO2 can be used to store renewable electric energy in the form of the well-known transportable gaseous energy carrier methane, which makes it a crucial technology to transform our existing fossil-based energy infrastructure. Methanation plants suitable for industrial-scale capacity at low complexity are necessary to produce cheap renewable synthetic natural gas (SNG) from fossil or biogenic carbon sources. To achieve highly optimized methanation reactors for CO/CO2 feed, such as from high temperature Co-electrolysis (Co-SOEC), the limiting mechanisms of existing reactor systems were identified. Several reactors with different dimensions, natural air-cooling or thermal-oil cooling and under variation of process conditions including pressure and catalyst load were experimentally investigated. A 1D plug-flow reactor model in MATLAB and a 2D CFD reactor model in COMSOL Multiphysics were developed and used to verify experimental findings and deepen the understanding of the methanation process. Based on a combined modelling and experimental approach tuning parameters were derived to overcome process limitations and a strategy to design high performance methanation reactors was elaborated. Thermodynamic and kinetic limitations along the reactor axis could be identified and significantly reduced by optimizing the axial temperature curve based on appropriate reactor dimensions and operation parameters. The 1D model works as a reactor optimization and design tool for CO/CO2 mixtures and other feed gases, such as biogas or CO2. Reactor and process design examples for single-stage Co-SOEC syngas methanation, high-capacity methanation at 100.000 h-1 GHSV (gas hourly space velocity) and energy efficient dual-pressure stage methanation are presented. The findings and proposals formulated in this thesis significantly improved the process of CO/CO2 methanation with the aim to contribute to a liveable future for upcoming generations on our planet.