Fluctuating wind and solar energy can be used to produce hydrogen by water electrolysis and subsequently for the synthetic natural gas production via methanation in the power-to-gas process. This paper investigates the unsteady-state operation of the methanation in an adiabatic and cooled fixed-bed reactor, respectively, with product recirculation by simulation of a one-dimensional fixed-bed reactor model. The results show that adiabatic fixed-bed reactors with product recirculation can operate in a wide range of partial and excess load. The recirculation of product gas cools the adiabatic fixed-bed reactor effectively and an optimal recycle ratio for the highest methane productivity exists. Cooled reactors are very sensitive to load changes of the volumetric flow rate and thus less flexible. However, the recycle of product gas allows reducing the sensitivity for a more stable operation under fluctuating feed conditions. The start-up time of cooled fixed-bed reactors is considerably lower. In summary, the flexibility of the dynamic methanation is enhanced in a loop-reactor arrangement.