The copolymerization of ethylene with small amounts of 1,9-decadiene can lead to massive crosslinking of the polyethylene resin and severe fouling of commercial reactors. This work is a first step towards developing the manufacturing know-how to reliably produce 1,9-decadiene/ethylene copolymers. We have developed a gel-point model to predict the maximum amount of 1-9-decadiene, permitting fouling-free operation of well-mixed reactors. The gel-point model relies on three reactivity ratios to characterize the response of a catalyst system. A series of 1,9-decadiene copolymers has been synthesized in a continuous stirred-tank reactor to determine the reactivity ratios for a metallocene catalyst, and to validate the model predictions for the polymer properties and for the onset of fouling. The experimental results indicate that the double bonds in 1,9-decadiene are, on average, as reactive as those found in alpha olefins of similar molecular weight, and that the reactivity of 1,9-decadiene double bonds decreases after the 1,9-decadiene molecules become part of a polymer chain. The gel-point model predictions of polymer properties agree well with chromatographic, density, and mass-balance data, and the gel-point model was successful in preventing unintended fouling during the duration of the experimental campaign.