Carbon formation reactions:

CH4 2H2 + C  (methane decomposition)

2CO CO2 + C (CO disproportionation)

The carbon formed deactivates the catalyst.

I'm not sure coke formation in dry cracking is at equilibrium but probable reaction is:

CH4 (s)---> C(s) + 2H2(g)

After a while coke formation will result in tube blockage or catalyst deformation

Yes, the carbon is in solid form, while hydrogen and methane are gases.

I based my answer in this paper:

http://www.sciencedirect.com/science/article/pii/S0378775310019701

After your answer I looked some other documents to check, like this one:

http://sti.srs.gov/fulltext/ms2001249/ms2001249.html

But in the reactions conditions of dry reforning there will be an accumulation of coke (carbon) on the surface of the catalyst and this will result in the deactivation of the catalyst and in some cases reactor blockage. So, the reaction is going to the direction of carbon formation in dry reforming.

Oh mistaken

Methane is in gas state but I mentioned solid.