A method of monitoring multiphase fluid flow passing within a pipe, which multiphase fluid flow includes a gas phase and a liquid phase, the method comprising the steps of: providing a flow pressure value and a flow temperature value for the multiphase fluid flow within the pipe; providing a fluid flowmeter operable to be attached to an exterior of the pipe, the flowmeter including a spatial array of at least two sensors disposed at different axial positions along the pipe, which flowmeter is configured to produce flow velocity signals indicative of a velocity of the fluid flow traveling within the pipe; characterized by providing a processor configured to include an equation of state model for the pressure, volume, and temperature properties for the multiphase fluid flow, and further configured to receive hydrocarbon composition data values for the multiphase fluid flow, the flow pressure value, and the flow temperature value, and the flow velocity signals from the flowmeter; determining a volumetric flow rate of one or both the gas phase and liquid phase of the fluid flow; wherein the fluid flow meter is a first fluid flowmeter that is configured to produce first flow velocity values, and wherein the method further includes the steps of: providing a second fluid flowmeter that is configured to produce second flow velocity values; determining a measured value for a fluid flow parameter using the first flow velocity values and the second flow velocity values; determining a calculated value for the fluid flow parameter using the flow pressure value and the flow temperature value and the equation of state model; comparing the measured and calculated values of the fluid flow parameter to determine an error value; determining modified hydrocarbon composition data values using the current hydrocarbon composition data values if the error value exceeds a predetermined value, and iteratively repeating the steps of determining the calculated value, comparing the measured and calculated values, and determining modified hydrocarbon composition data values until the error value is less than the predetermined value; and determining the volumetric flow rate of one or both the gas phase and liquid phase of the fluid flow using the modified hydrocarbon composition data values. providing a flow pressure value and a flow temperature value for the multiphase fluid flow within the pipe; providing a fluid flowmeter operable to be attached to an exterior of the pipe, the flowmeter including a spatial array of at least two sensors disposed at different axial positions along the pipe, which flowmeter is configured to produce flow velocity signals indicative of a velocity of the fluid flow traveling within the pipe; characterized by providing a processor configured to include an equation of state model for the pressure, volume, and temperature properties for the multiphase fluid flow, and further configured to receive hydrocarbon composition data values for the multiphase fluid flow, the flow pressure value, and the flow temperature value, and the flow velocity signals from the flowmeter; determining a volumetric flow rate of one or both the gas phase and liquid phase of the fluid flow; wherein the fluid flow meter is a first fluid flowmeter that is configured to produce first flow velocity values, and wherein the method further includes the steps of: providing a second fluid flowmeter that is configured to produce second flow velocity values; determining a measured value for a fluid flow parameter using the first flow velocity values and the second flow velocity values; determining a calculated value for the fluid flow parameter using the flow pressure value and the flow temperature value and the equation of state model; comparing the measured and calculated values of the fluid flow parameter to determine an error value; determining modified hydrocarbon composition data values using the current hydrocarbon composition data values if the error value exceeds a predetermined value, and iteratively repeating the steps of determining the calculated value, comparing the measured and calculated values, and determining modified hydrocarbon composition data values until the error value is less than the predetermined value; and determining the volumetric flow rate of one or both the gas phase and liquid phase of the fluid flow using the modified hydrocarbon composition data values. providing a second fluid flowmeter that is configured to produce second flow velocity values; determining a measured value for a fluid flow parameter using the first flow velocity values and the second flow velocity values; determining a calculated value for the fluid flow parameter using the flow pressure value and the flow temperature value and the equation of state model; comparing the measured and calculated values of the fluid flow parameter to determine an error value; determining modified hydrocarbon composition data values using the current hydrocarbon composition data values if the error value exceeds a predetermined value, and iteratively repeating the steps of determining the calculated value, comparing the measured and calculated values, and determining modified hydrocarbon composition data values until the error value is less than the predetermined value; and determining the volumetric flow rate of one or both the gas phase and liquid phase of the fluid flow using the modified hydrocarbon composition data values.