in the surface ribbon, there is a sew surface tool. (it may be hidden and you will need to turn it on in the ribbon)
select this tool and select one of the surfaces.
select all the other surfaces that you want united.
hit the preview option to see if/where you have errors (you will most likely have errors)
if you are certain that you are not missing a sliver and it still refuses to sew, you can change the tolerance to a higher value and hit preview again. increase the tolerance in small increments, too high of a number can cause issues too.
once preview is clean, hit ok
Would it be possible to avoid using the IGES format ?
The IGES standard is an old and outdated file format.
The last (!) published standard is 5.3 which was published 1996. 25 years ago.
If the systems you work with can use STEP / JT / Parasolid instead you might not see these issues at all.
Regards,
Tomas
Never try to teach a pig to sing. I wastes your time and it annoys the pig.