A mathematical model and numerical simulations are presented for an electrochemical reactor for the production of fluorinated organic compounds. The liquid-fed, vertical flow system is complicated by the generation of hydrogen gas byproduct. The one-dimensional model for this continuous-flow, multiple-cell modification of the Simons process extends gas-liquid flow regimes, thermal-energy considerations, and phase equilibrium developed earlier by Drake et. al. (J. Electrochem. Soc. 1998, 145, 1578). Association of a reactant, HF, in the gas phase is also modeled and is important. For a reactor with a single cell or multiple identically functioning cells, the production rate (total current) undergoes a maximum at 14.5 V/cell for a given inlet flow rate, inlet temperature, inlet composition, and exit manifold pressure. Beyond the maximum, inefficient operation results, with a decreasing production rate as applied potential increases. Electrically resistive, non-liquid-phase-continuous flow occurs in an increasing portion of the reactor due to high gas-generation rates. Conditions for inefficient multiple steady states are proposed, for given reactor specifications.