The lateral growth of polycrystalline clathrate-hydrate films along the interface between liquid water and a hydrate-forming substance in the state of a liquid or a gas is considered to be rate-controlled by the transfer of heat of hydrate formation from the edge of each film to the surrounding, i.e., the water and hydrate-former phases. A simple convective-heat-transfer model is presented, which predicts the product of lateral growth rate of a film and its thickness to be proportional to the 3/2 power of the effective temperature driving force. The application of the model to the existent experimental data on the growth of CO2-hydrate films leads to a rough estimation of their initial thickness, 0.3-0.6 mum.