The firm successfully completed operational trials of the Train 2 unit ahead of it receiving shale gas ethane from the US this autumn.
New docks and a new pipeline network have been built to receive the gas.
The company said the move means it is now in "great shape" to run the plant at full capacity.
The KG ethylene cracker was unable to operate at full capacity in 2008, leaving Ineos "no option" but to close the second manufacturing unit.
Ineos said the US ethane will be used as a supplementary feed for the KG ethylene plant and will allow the plant to run at increased rates.
Gordon Milne, Ineos Grangemouth operations director, said: "With the successful completion of the Train 2 trial we are now in great shape to receive shale gas from the US and to finally run the Grangemouth plant at full rates.
"All the parts of the jigsaw are finally coming together and Grangemouth will soon be back in the premier league of European petrochemical plants."
The plant's chief executive John McNally said: "Bringing the site back into profitability is the best way to secure our future here in Scotland.
"We know that ethane from US shale gas has transformed US manufacturing and we are now a step closer to seeing this advantage being brought to here to Grangemouth."
A second manufacturing unit at the Ineos KG ethylene plant in Grangemouth will be brought back to life eight years after it was mothballed.