A method to control reactor split and a product parameter of a polyethylene product in a multimodal polymerization reaction comprising: a) polymerizing at least one monomer in a gas phase reactor in the presence of a supported multimodal catalyst system comprising at least two different catalyst compounds to form a multimodal polyethylene product having a reactor split equal to respective weight fractions of resin components in the polyethylene product; b) applying a predetermined formula for a product parameter of the multimodal polyethylene product, wherein the product parameter is flow index and the predetermined formula is Flow Index = e{ A+C[H2 mol:C2 mol]+D[C6 mol:C2 mol]+E(ICA pp)} * (HMW mol: LMW mol) B, wherein A is an intercept determined from a best fit line, and B, C, D, and E are slope terms determined from the best fit line,H2 mol:C2 mol is the molar ratio of hydrogen to ethylene, C6 mol:C2 mol is the molar ratio of hexene to ethylene, ICApp is the induced condensing agent partial pressure, and HMW mol: LMW mol is the molar ratio of the high molecular weight fraction to the low molecular weight fraction; c) obtaining incorporation data and production rate data from the reaction based upon the predetermined formula; d) determining an actual hydrogen leading indicator, wherein the hydrogen leading indicator is a H2/C2 flow ratio; e) comparing the actual hydrogen leading indicator to a target value for a hydrogen leading indicator to determine a deviation of the actual hydrogen leading indicator from the target value for the hydrogen leading indicator; and f) adjusting an amount of a catalyst precursor for forming at least one of the two different catalyst compounds and being fed to the gas phase reactor to control reactor split equal to respective weight fractions of resin components in the polyethylene product and the product parameter of the polyethylene product wherein the catalyst precursor is activated and forms at least one of the catalyst compounds of the supported catalysts in the gas phase reactor and wherein the adjusted amount of catalyst precursor is based upon data obtained in c) and the deviation determined in e). a) polymerizing at least one monomer in a gas phase reactor in the presence of a supported multimodal catalyst system comprising at least two different catalyst compounds to form a multimodal polyethylene product having a reactor split equal to respective weight fractions of resin components in the polyethylene product; b) applying a predetermined formula for a product parameter of the multimodal polyethylene product, wherein the product parameter is flow index and the predetermined formula is Flow Index = e{ A+C[H2 mol:C2 mol]+D[C6 mol:C2 mol]+E(ICA pp)} * (HMW mol: LMW mol) B, wherein A is an intercept determined from a best fit line, and B, C, D, and E are slope terms determined from the best fit line,H2 mol:C2 mol is the molar ratio of hydrogen to ethylene, C6 mol:C2 mol is the molar ratio of hexene to ethylene, ICApp is the induced condensing agent partial pressure, and HMW mol: LMW mol is the molar ratio of the high molecular weight fraction to the low molecular weight fraction; c) obtaining incorporation data and production rate data from the reaction based upon the predetermined formula; d) determining an actual hydrogen leading indicator, wherein the hydrogen leading indicator is a H2/C2 flow ratio; e) comparing the actual hydrogen leading indicator to a target value for a hydrogen leading indicator to determine a deviation of the actual hydrogen leading indicator from the target value for the hydrogen leading indicator; and f) adjusting an amount of a catalyst precursor for forming at least one of the two different catalyst compounds and being fed to the gas phase reactor to control reactor split equal to respective weight fractions of resin components in the polyethylene product and the product parameter of the polyethylene product wherein the catalyst precursor is activated and forms at least one of the catalyst compounds of the supported catalysts in the gas phase reactor and wherein the adjusted amount of catalyst precursor is based upon data obtained in c) and the deviation determined in e).