Hello Zara, apologies for not replying sooner but I completely missed your post for some reason.
Generally speaking the central support stanchion located behind the fireplace should not be removed as it it is the central support for the entire upper floor of the house.
The majority of open plan conversions that we see attempt to incorporate the central stanchion into the design with varying success. That is not to say that you can't undertake what you are proposing but it would almost certainly require the services of a structural engineer to devise a solution that would not undermine the integrity of the exisiting frame. Any support structure would most likely need to be self supporting as it probably would not be connectable to the existing frame. You would also need to get this work approved by your local planning department, which from my experience, is easier said than done when removing portions of the existing steel framework.
Here's a few links to previously asked questions and posts regarding open plans.