Power-to-Gas (P2G) plays an important role in enhancing large-scale renewable energy integration in power systems. As an emerging inter-disciplinary subject, P2G technology requires knowledge in electrochemistry, electrical engineering, thermodynamic engineering, chemical engineering and system engineering. Aiming at P2G modeling and operational problems concerning the research field of power systems and the energy internet, this paper briefly reviews the main technologies and application potentials of the P2G system, and makes systematic summaries of major progresses related to P2G's integration into the power grid in a bottom-top manner, including the modeling of high/room-temperature electrolysis cells, steady-state/dynamic optimization control of the P2G system, P2G's integrated model and operational strategies at the grid level. In the final part of this paper, suggestions are put forward on future research directions of P2G systems from the aspects of modeling and operational optimization.