Thermocatalytic decomposition (TCD) of methane can be used to produce pure hydrogen and valuable nanocarbons without emitting carbon dioxide. In this report, the thermocatalytic decomposition of methane in a fluidized bed reactor is modeled. The hydrodynamics for the three-phase bubbling fluidized bed are taken from the book of Kunii and Levenspiel. This is combined with literature on the kinetics of the decomposition and deactivation reactions. The catalyst is modeled with a separable kinetics approach in which the TCD reaction is time-independent and the deactivation of the catalyst is time-dependent. The optimum operating conditions are found for a catalyst residence time of 1 day and 1 week. It is found that the optimum temperature is always the maximum temperature in the range, whereas the optimum superficial gas velocity increases for higher reactor diameters. For a residence time of 1 week, hydrogen should be added for small reactor diameters. It is found that the TCD reaction is not competitive with the steam-methane reforming reaction in terms of reactor sizing. Results are also generated for kinetics in a fluidized bed reactor. These results are however limited to a narrow range of hydrogen partial pressures.