The growth of variable renewable energy sources requires new solutions for a secure and continuous energy supply. The keys are innovative energy storage systems, where Power-to-Gas (PtG) is one promising concept. To respond to the transient energy production, a flexible PtG process chain is necessary. This thesis investigates the feasibility and limits of partial-load behaviour in a fluidized-bed methanation reactor as part of an innovative process design for biogas upgrading plants. The vision is a year-round operation in different utilization stages. To imitate those stages, different scenarios (0 % methanation determining the maximum requirement for the membrane unit, 100 % methanation determining the maximum requirement for the reactor and various partial loads using the given dimensions) are simulated in a MATLAB-based toolbox for an industrial-scale 200 Nm3/h plant consisting of a membrane unit and a methanation reactor as the main parts. After validating the membrane model, the objective is to find the right process parameters (e.g. number of membrane modules and reactor pressure) to meet the grid injection requirements. Different membrane properties and gas velocities are tested, and the process management is varied in terms of the number of compressors in use. In addition, proof-of-concept experiments are conducted to show the feasibility of full and partial load in a fluidized-bed methanation reactor. Taking a well-established biogas upgrading plant as reference, the membrane model could successfully be validated. The determined number of membrane modules from the 0 % methanation scenario is sufficient for the full- and partial-load PtG scenarios to also meet the grid injection requirements. With the given reactor parameters from the full PtG scenarios, the minimum partial load was determined at 20 and 30 % respectively, depending on the membrane material and the number of compressors. If the chosen process parameters were not sufficient for the simulation and the grid requirements, a reduction in the number of membrane modules and/or the reactor pressure provided remedy. Using a higher system pressure, the full PtG scenario with one compressor requires the same number of membrane modules as the 0 % methanation scenario. For the experiments with a recycle stream, the minimum partial load was at 40 % while still meeting the grid injection requirements.