Not necessary, because the argument isn’t based on a comparison of expenses for natural gas and expenses for other things.

Necessary, because if it were not true — if the price of natural gas in Tilsen WILL increase — then we cannot infer that any cost increase in Chester will make the cost of natural gas more than twice as much as the cost in Tilsen. And in that case we cannot conclude that any increase in cost of gas in Chester would lead to movement of manufacturing operations.

Not necessary, because the argument isn’t based on any claims concerning the profitability of ChesChem.

Not necessary, because whether there are other benefits to ChesChem moving to Tilsen is irrelevant. Other benefits don’t affect the reasoning, which is based on a premise that indicates a condition under which ChesChem will move. That condition concerns only the ratio of cost of natural gas in Chester and Tilsen.

Not necessary, because this confuses sufficient and necesary conditions. The author believes that an increase in cost of natural gas in Chester is SUFFICIENT for ChesChem to move to Tilsen. That doesn’t imply the author believes that it’s necessary in order for ChesChem to move to Tilsen. ChesChem may still move to Tilsen even if the cost of natural gas doesn’t increase in Chester, and that would not undermine the argument.