A monitoring system and method monitors the gas flow rate, the liquid flow rate, and the water cut of liquid in a multi-phase petroleum stream which includes stratifying the petroleum stream by having it flow down a decline at a predetermined angle to the horizontal. The stratified stream flows into a gas trap from which the liquid exits by a liquid exit pipe which is connected to another section of the pipeline. A gas accumulator is connected to the stratifying pipe and to the gas trap and collects the gas from them. A gas exit pipe is connected to the accumulator and to the other section of the pipeline and allows the gas to flow from the accumulator to the pipeline. A gas flow rate meter is connected to the gas exit pipe and provides a signal corresponding to the gas flow rate. A liquid flow rate meter is connected to the liquid exit pipe and provides a liquid flow rate signal. A water cut meter is connected to the liquid exit pipe and provides a signal corresponding to the water cut of the liquid exiting the trap.