Pre-plant application of toxic fumigants to soil beds covered by plastic film is commonly used in agriculture to control soil-borne pathogens. Plastic mulch covers tend to physically suppress the emissive loss of gaseous fumigant to the atmosphere. When liquid fumigant metham sodium (MS) is applied in irrigation water to field soil, it is rapidly transformed to the gaseous methyl isothiocyanate (MITC). The gaseous MITC is a potential atmospheric contaminant, and any untransformed MS is a potential contaminant of underlying groundwater due to the high water solubility of MS. A finite element numerical model was developed to investigate two-dimensional MITC fate/transport under non-isothermal soil conditions. Directional solar heating on soil beds, coupled heat and water flow in the soil, and non-isothermal chemical transport were included in the model. Field soil data for MITC distribution, soil water content, meteorological data, and laboratory data were used to verify the model for soil beds covered with plastic mulch. Four possible scenarios were considered: low and high drip-irrigation rates and low and high water contents. The movement of the center of MITC mass in the soil profile was effectively simulated. The lower drip-irrigation rate of MS yielded more extensive coverage of MITC in the plastic-covered soil bed. The lower soil air contents due to higher soil water contents for the higher irrigation rate resulted in high concentrations of soil MITC. NRMSE (normalized root mean square error) calculations further verified that the model predicted fumigant fate/transport well under these non-isothermal field conditions.