Purpose of Porcelain Sealer
In new porcelain tile installations or when new grout is being applied, it is recommended that you apply a penetrating sealer that impregnates the grout with polymers in order to prevent staining of the grout. The sealer is meant to be applied only to grout, not on the porcelain tiles themselves. You should mask off baseboards and other surfaces to which you don't mean to apply sealer, to protect them from overspray and splashing. Still, the sealer often gets on the tiles themselves. If allowed to dry, the sealer can leave a hazy plastic residue on the tiles because of the polymers lying on the tile surface.
If the Sealer Is Still Wet
During installation if the sealer is still wet, manufacturers say all you need to do is wipe or mop the entire surface of the tile with a damp, lint-free soft cloth or mop. The sealer doesn't damage the tiles but if it dries it can make them look ugly--definitely not the look you were after. As long as you clean it right away the sealer won't stick to the porcelain.
If the Sealer Has Dried
Perhaps your floor was installed while you were out, or if you did it yourself, you didn't realize you'd gotten sealer on the tiles. You've got that plasticky residue dulling your beautiful tiles. Underneath, however, your tiles are not damaged. Now it's time to purchase sealer remover from the manufacturer, if you know who made your tiles. If you don't know, your local home repair center should be able to recommend a generic sealer remover that may help.